Apparatus for preventing radio interference



April 9 w. c. HALL 2,466,311

APPARATUS FOR PREVENTING RADIO INTERFERENGES Original Filed Jan. 18, 1944 gwvmvlm WAYNE C. HALL.

ATTORNEY Patented Apr. 5, 1949 APPARATUS FOR PREVENTING RADIO INTERFERENCE Wayne 0. Hall, Cheverly, Md.

Continuation of application Serial No. 518,692, January 18, 1944. This application February 3, 1947, Serial No. 726,194

(Granted under the act of March 3, 1883, as amended April 30, 1928; 370 0. G. 757) 6 Claims.

This application is a continuation of application 518,692 filed January 18, 1944, now abandoned.

This invention relates to a method and apparatus for reducing interference to radio reception in the vicinity of highly charged bodies, and it is particularly concerned with the reduction of interference to reception onaircraft in flight.

It is well known that large metallic bodies, such as ships, aircraft and other structures, become highly charged when in the vicinity of electrical storms or certain types of wind storms such as dust storms, or drifting snow. Aircraft in particular accumulate very high charges when traveling through certain types of clouds. The large electrical fields about such bodies cause corona discharges to take place from those parts of the body which are subject to the strongest electrical fields, such as masts of ships, wing tips, propellers and tails of aircraft and the like. This corona discharge produces an extremely noisy interference in any nearby radio receiving equip- 'ment, often causing complete failure of communication at times when such communication is vital. For example, an aircraft pilot flying through storm areas quite often meets these conditions in an overcast where the temperature is near freezing, and he may find himself unable to receive beam or other radio signals at a time when he must depend on instruments alone for his course.

In copending application, Serial Number 469,- 606, filed December 21, 1942, there is described a method and apparatus for facilitating the dis charge of electricity from charged bodies in a manner which causes little or no radio interference, by conducting the discharge into the atmosphere from the fibers of a non-conducting material which has been rendered semi-conducting (i. e, of high resistance). A suitable means for rendering the material semi-conducting is described as moistening the material with a liquid of suitable electrical conductivity.

The instant invention provides a method of facilitating the discharge of electricity from charged bodies in a manner which causes little or no radio interference and it includes novel apparatus for accomplishing the same. Due to the higher discharge rate obtainable without appreciable radio interference by the method of this invention (as compared to the noisy discharge obtained under ordinary conditions), the electric fields about the body affected are maintained below that necessary to cause noisy corona 2 discharges. In this way the method and apparatus herein disclosed permit radio communication under conditions heretofore difficult, and often impossible, by reducing the interference caused by corona discharge to a negligible amount.

The method of this invention comprises causing the electricity to discharge into the atmosphere in a noise-free manner from one or more selected parts of the charged body at an electric field too low to initiate noisy discharge from the major portion of the body. If necessary, additional steps are taken to prevent discharges from other exposed parts of the body. The discharging means must operate to conduct electricity from the body before other parts thereof break into corona, and in a manner which does not create appreciable electrostatic or electromagnetic disturbances. The method here employed is based on the discovery that electricity can be discharged into the atmosphere with only a negligible amount of electrostatic and electromagnetic disturbance, if it is discharged from a fibrous surface, or area, of a semi-conducting nature. These surfaces are prepared from non-conducting fibrous materials by any of several means in addition to those disclosed in the forementioned application. If synthetic fibers are used, the plastic from which they are made may be rendered semi-conducting by incorporation therein of various amounts of finely dispensed conducting materials, such as finely divided metal or carbon. Another method, which is particularly applicable where the fibers are of the natural animal or vegetable type, consists in impregnating or coating them with microscopic metal particles (which, under some conditions, may form a metal film on the surface of the fibers). For best results the current should be discharged from a great number of fine fibers close together. A porous surface with a multiude of fine points projecting outward such as chamois skin, cotton, wool, glass and synthetic fibers, is satisfactory as a di-scharger when made conducting in the manner indicated. The preferred form involves the use of a fibrous rope or wick which has been rendered semi-conducting, one end of the wick being fastened to the charged body and the other end supported (mechanically, or by air flow) away from the body and extending into the atmosphere. In order that the discharge will preferentially take place through the wick and so avoid corona discharge from other parts of the body, it is necessary to fasten the wick (or electrode) to the body in a region where a high electrical field surrounds the charged body. In the case of surface craft such points would logically be the mast and other projections, and in aircraft it would be mainly the ends of the wings and the tail. The propellers also are areas of high electric field, but it is diillcult to place these discharges on them. At certain points or regions such as the ailerons where projecting dischargers might be objectionable a fibrous material, such as cotton flannel, suitably impregnated with metal particles (or otherwise made semi-conducting), may be used as a noise-free discharger by cementing it to the edge of the surface. Whenever a wick type discharge is used, which projects outwardly from the structure, the electrical shielding provided by the wick will be suiilcient to prevent discharge from adjacent areas over a distance equal approximately to the distance from the end of the wick to the surface of the structure. Under severe conditions a reasonable number of these wicks, or noise-free dischargers, may not keep the electric fields about the entire structure low enough to prevent a noisy discharge from certain edges particularly the exposed antenna systems. In this event the discharge is prevented from those edges or parts by insulating them, such as by painting or covering the edge, point or region with some material of high dielectric strength, and of high dielectric constant. Such a material may be any of several suitable synthetic resins, waxes or tars and other commercially available prepared materials such as a thin rubber sheet glued to the surface. When the point has been so covered, no discharge from the protected region or point will take place. Ceresin wax and beeswax have been found particularly effectiv in preventing discharge, but

have the disadvantage of cracking away from the metal surface at low temperatures. Synthetic resins of particular usefulness for this purpose are the vinyl and acrylic ester resins, including polystyrene, and phenol-formaldehyde and alkyd resins. For antenna systems, polyethylene resins are preferred because of their low loss properties and desirable physical characteristics at low temperatures. Combinations of layers of different materials such as synthetic resins and beeswax, or ceresin wax have been tried and found better than either ceresin or beeswax alone, but these multiple layers are prone to break away from the metal at low temperature. Various coating thicknesses may be used. However, experiment has shown that a layer approximately .030" to .040" thick offers sufllcient protection. This method of insulation for the prevention of noisy corona discharge is particularly applicable to the edges of the propellers.

Certain large insulated areas which are exposed to the weather, such as Windshields and de-icer boots of rubber, in contrast-to the very small, deliberately insulated areas described above, may cause objectionably loud static through charging and sparking over to the surrounding metallic parts of the plane, due to their size. Such static will be prevented, if the surface is painted with a conducting paint. A conducting paint suitable for the rubber de-icer boots may be made by mixing rubber cement and colloidal carbon with a suitable thinner, or it may be made by any of the well-known methods. with respect to the windshield a film of glycerol and water may be sprayed on from the outside by means of a pressure sprayer controlled from the cabin., Any of the conducting non-freezing liquids such as aqueous alcohol may also be sprayed on, but a liquid of low vapor pressure, such as glycerol and the glycols, is preferable to avoid undue loss through evaporation.

In the event that the type of storm through which the aircraft is flying involves water above its freezing point, the water itself will conduct much of the electricity away from the insulated area to the surrounding parts of the plane.

Several embodiments of the apparatus of this invention and their application to aircraft are described in detail with reference to the accompanying drawings in which:

Fig. 1 shows in simplified form one embodiment of the discharging device of this invention;

Fig. 2 shows the apparatus of Fig. l in a mounting;

Fig, 3 shows another embodiment of the discharging device of this invention, mounted at the edge of an airfoil; and

Fig. 4 is a top view of an aeroplane showing suitable placement of the devices illustrated in Figs. 1 through 3.

Referring to Figs. 1 and 2, a non-conducting tube In (e. g. a plastic tube) is fastened to a metal holder ll having mounting lugs l2 and I3 adapted for lock mounting in a fitment H. In the tube In is a standard wick l'5 (e. g. a inch lamp wick) which extends an inch or so from the end of the tube ID. The wick l5 has been impregnated with microscopic or colloidal particles of metal such as are deposited from chemical solutions of certain salts on treatment with suitable reducing agents. For example, platinum may be deposited from either chloroplatinic acid and gold may be deposited from chlorauric acid. Other well-known metal solutions may be used, and the preferred one is ammoniacal silver nitrate. The wick I! was soaked in about a 10% solution of ammoniacal silver nitrate and then treated with any reducing agent such as ferrous sulfate, sugar, etc., thus leaving finely divided silver throughout the wick. Any of the common silvering solutions, such as Brashears solution, may be used to deposit the fine silver particles in the fibrous material, although best results usually require somewhat higher concentrations of silver. The proportions are not critical, and the preparation of suitable metal-depositing solutions is well understood in the art.

In the actual impregnation of the wick or other fibrous material, it is not essential that the silver (or other metal) particles be uniformly scattered throughout, but the more nearly uniform the impregnation is, the more satisfactory is the operation of the material under discharge conditions.

The exposed end of the wick is fluifed out to provide as many separate fiber ends as possible for the discharge, as this permits a higher discharge rate without radio interference. The other end of the wick I5 is connected to the metal holder ll either directly by clamping or, preferably, by means of any conducting cement, such as suspensions of colloidal carbon in lacquers, cements, etc.

The resistance of such a wick may vary widely, from .5 to 20 megohms or more. The valu preferable for most purposes is around 1 megohm. The body of the lamp wick described is formed of unbonded, inter-twisted fibers. It is necessary, of course, that the discharge points be separate free fibers. Chamois skin, mentioned above, is an example of a matted and bonded fibrous substance providing free fibers at its surface. The semiconductive fluid impregnated wicks may ofler another example in which the body of the wick carries suflicient liquid so that its conduction is substantially isotropic but terminating in free discrete fibers, semi-conductive only along their length. In case a wick of glass fibers is impregnated with a chemically deposited metal, the latter will of necessity not penetrate the individual fibers but will lie on their surfaces. Such deposit, even if continuous, may be so thin as to be only semi-conductive.

The metal on the fibers of the Wick or other material may be in the form of discrete particles or in form of an extremely thin, continuous film made up of adjoining or overlapping particles. Although there is no particular lower limit to their size, these particles must not be so large as to be visible to the naked eye and ar therefore referred to as microscopic. If the particles should be visible to the eye, the discharge therefrom would cause noise in nearby radio receivers.

The electrical characteristics of the wick carrying microscopic precipitated metallic particles depends on their physical character and arrangement. Different conditions are tolerabl in different parts of the wick. In the interior of tube In (Fig. 1) over-all high resistance is desired. Since corona discharge to the atmosphere does not occur, large particles would not cause interference here. The exposed fibers carrying d scharge currents must be devoid of visible highly conductive bodies, and the discharge surface is therefore Wholly semi-conductive macroscopically. The portions of the wick from which discharge does not occur may include particles of greater than microscopic size, of substantial conductivity, without rendering the discharge noisy, but high over-all resistance between such bodies and the dicharge surface is desirable. Since interior wick portions may through the course of use become exposed and constitute discharge surfaces, the presence of such bodies is not desirable.

In the application of the present inventionv to the needs of military aviation in particular, a great deal of analysis of its operation has been effected, together with a study of the physical principles involved in the charging and discharging of aircraft. In this connection the papers published in Proceedings: Institute of Radio En= gineers, vol 34, pages 156, 161, 157, 175, and 234 may be referred to.

It was found that the effectiveness of the wick discharger is due to a number of factors. Initially it may be stated that the dry wick dischargers of the instant application operate on the same principles as those described in application S. N. 469,606. The latter consist of wicks impregnated with a semi-conductive, or highly resistive, liquid. The wickmust be dampened only, and as explained in that application, free liquid surfac at the end of the wick renders the discharge noisy.

Silent discharge is obtained by a high resistance path from the aircraft terminating in a multiplicity of sharp discharge points which themselves are highly resistive. that this discharge comprises a continuous but very small current from each point. Highly conductive particles of even minute size, although sharp and effecting discharge fromsurfaces of very small radius of curvature, give rise to noise. Highly resistive surfaces of comparatively large radius of curvature will also give rise to noise. It is believed these latter generate noise by intermittent discharges.

The intermittent ionic discharge through which such undesired conduction takes place appears to generate interference in an amount de- It is believed I pending to a substantial extent upon the energy transferred. This in turn depends upon the energy available, and in the case where resistive decoupling from the plane is employed, to an approximation this is proportional to CE. C refers to the capacity of the body discharging as seen from the corona point, and E to the potential. This discharge from any one point is believed to be intermittent.

From the description of the wicks in the embodiment described, it may be inferred that the capacity of a particle of precipitated silver sufficiently large to be visible is enough to render the discharge of its substantially unipotentlal surface sufficiently intense to generate interference.

On this basis, the inefficiency of the single wir discharger of the prior art is apparent. The capacity of the metallic conductor as seen from the corona point is so great that its discharge generates a great deal of interference.

From the description of the noise generated by a free semi-conductive or highly resistive liquid surface in application S. N. 469,606, it is now believed that the potential to which the surface rises, before discharge, caused a corresponding increase in the energy available; it being understood of course, that'the corona potential is a function in part of the radius of curvature.

In some cases of dry Wicks carrying metallic particles of visible size, both factors may be present if a rounded particle of substantial size exists. Normally, however, such particles present unipotential surfaces of very irregular contour whose discharge potential depends on the sharpest point thereof. To avoid intermittent discharge or to reduce the noise level generated thereby to a negotiable amount..the dischargershould therefore provide a highly resistive final path to the atmosphere, and should terminate in a multiplicity of fine points. The wicks described provide a very efficient practical embodiment of the invention. By' their use discharge is effected by leading the charge to the atmosphere through a highly resistive path which is at least terminally divided into a multitude of fine filar points. The discharge is to be effected only from surfaces of very small radius of curvature, in the absence of substantial capacity as seen from the discharge point.

In Fig. 3 a cross-sectional view of an edge 30 at which corona discharge may take place is shown with a layer of cloth 3| or other fibrous material cemented in place at spots 32. The cloth. of course, has been impregnated with metal particles, such as silver, in the manner described with reference to the wick 15 of Figs. 1 and 2, Discharge from this form of discharger may be facilitated by havingone piece of material project into the air in the formof a number of small strips or ribbons (see Fig. 4). The distance back from the edge which the cloth must extend to permit substantially noise-free discharge is not great, one-half to 0ne-inch generally being sufficient.

It is clear that the above described embodiments of the invention may be used in a number of different ways. However, a typical application of them to an aircraft is shown in Fig. 4. In Fig. 4 a metal plane 40 is provided with electrodes or dischargers 4|, Q2 and 43, illustrated in Figs. 1 and 2 at the wing tips and tail. Cloth strip type of dischargers 44, 45, and 46, illustrated in Fig. 3, are provided at the trailing edges of the ailerons and elevators with small chamois tabs 41 extending, say, a couple of inches from the latter.

The plane 40 is provided with wing de-icers l8 and 49 which are regular inflatable rubber boots. These boots are painted with a conducting paint (such as may be prepared by mixing graphite or other forms of carbon, or finely dispersed metal, in a vehicle) to prevent spark discharge to the plane 40.

Since it is difficult to place dischargers on the tips 50 and SI of the propellers, the latter may be insulated by applying a coating of hard-baked insulating varnish or an air-drying insulating paint or varnish.

The design of the electrode may be modified to suit each particular installation. However,

. for attachment to parts of aircraft the end of the wick should extend at least several inches from the body of the aircraft for maximum discharge from the wick, and where the electrode is fastened so as to protrude from a surface of the aircraft, it would preferably extend about a foot to permit efilcient electrical discharge at comparatively low potentials. The optimum number of electrodes, their shape and their precise distribution are matters of design in each case, but the principles to be followed have been herein described and radio interference can be reduced accordingly without further experimentation.

Other variations will be apparent to those skilled in the art and the invention should not be limited other than as defined by the appended claims.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

What is claimed is:

1. In combination with a conductive surface of an aircraft for the purpose of discharging static electricity from said surface, a discharge element conductively secured to said surface and comprising an exposed mass of normally nonconductive fibrous material having myriad pro- Jecting fibers with free ends having radii of curvature of microscopic order, said fibers carrying highly resistive films of conductive material which films have in the area of said free ends a thickness of microscopic order.

2. In combination with a conductive surface of an aircraft for the purpose of discharging static electricity from said surface, a discharge element conductively secured to said surface and comprising an exposed mass of normally nonconductive fibrous material having myriad projecting fibers with free ends having radii of curvature of miscroscopic order, said fibers carrying highly resistive films of conductive solid mateterial which films have in the area of said free ends a thickness of microscopic order.

3. In combination with a conductive surface of an aircraft for the purpose of discharging static electricity from said surface, a discharge element conductively secured to said surface and comprising an exposed mass of normally nonconductive fibrous material having myriad projecting fibers with free ends having radii of curva- .5 Number 3 ture of microscopic order, said fibers carrying highly resistive films of finely dispersed particles of conductive solid material which films have in the area of said free ends a thickness of microscopic order.

4. In combination with a conductive surface of an aircraft for the purpose of discharging static electricity from said surface, a discharge element conductively secured to said surface and comprising an exposed mass of normally nonconductive fibrous material having myriad projecting fibers with free ends having radii of curvature of microscopic order, said fibers carrying highly resistive films of colloidal particles of conductive material which films have in the area of said free ends a thickness of microscopic order.

5. In combination with an aircraft, means for preventing electrical discharge from certain portions of the conductive surface of said aircraft in regions of high electrical field comprising a coating of non-conductive material overlying said surface, and means for providing discharge from another portion of said aircraft comprising a discharge element conductively secured to a conductive surface at said other portion of said aircraft and including an exposed mass of normally non-conductive fibrous material having myriad projecting fibers with free ends having radii of curvature of microscopic order, said fibers carrying highly resistive films of conductive material, which films have in the area of said free ends a thickness of microscopic order.

6. In combination with an aircraft, means for preventing electrical discharge from certain portions of the conductive surface of said aircraft in regions of high electrical field comprising a coating of non-conductive material everlying said surface, and means for providing discharge from another portion of said aircraft comprising a discharge element conductively secured to a conductive surface at said other portion of said aircraft and including an exposed mass of normally non-conductive fibrous material having myriad projecting fibers with free ends having radii of curvature of microscopic order, said fibers carrying highly resistive films of conductive solid material, which films have in the area of said free ends a thickness of microscopic order.

WAYNE C. HALL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Date Clark Oct. 16, 1917 Howard June 13, 1922 Cadwell Nov. 17, 1942 George Jan. 26, 1943 Thacker May 4, 1943 Leake May 25, 1943 FOREIGN PATENTS Country Date Great Britain 1913 Great Britain Aug. 28, 1942 Number Certificate of Correction Patent No. 2,466,311 April 5, 1949 WAYNE O. HALL It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

Column 8, line 37, for everlying read overlying and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Ofiice.

Signed and sealed this 21st day of March, A. D. 1950.

THOMAS F. MURPHY,

Assistant C'ommz'ssz'omr of Patents.

, Certificate of Correction Patent No. 2,466,311 April 5, 1949 WAYNE O. HALL It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

Column 8, line 37, for overlying read overlying and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 21st day of March, A. D. 1950.

THOMAS F. MURPHY,

Assistant G'ommz'ssz'omr of Patents. 

